Fractionation of c12 bicyclic aromatic hydrocarbons by di or trianhydride complex formation

ABSTRACT

Mixtures of C12 bicyclic aromatic hydrocarbons containing dimethylnaphthalenes are difficult to fractionate by conventional techniques such as distillation or crystallization. However, by contacting such mixtures with the dianhydride of 1,2,4,5benzenetetracarboxylic acid, the dianhydride of 1,2,3,4benzenetetracarboxylic acid or the trianhydride of 1,2,3,4,5,6,benzenehexacarboxylic acid, a solid complex of certain hydrocarbons and the polyanhydride is formed. Separation of the solid complex and its subsequent decomposition results in a complexate that is substantially richer in those dimethylnaphthalenes which are preferentially complexed. Upon further processing these dimethylnaphthalenes have utility in the production of dyes.

O United States Patent [151 3,665,043

Dav s t a 51 May 23, 1972 FRACTIONATION OF C BICYCLIC OTHER PUBLICATIONS AROMATIC HYDROCARBONS BY 1)] Rosenberg et al., Journal of Physical Chemistry, Vol. 70, OR TRIANHYDRIDE COMPLEX Pages 4096* FORMATION Primary ExaminerDelbert E. Gantz [72] Inventors: Ronald 1. Davis, Wilmington, Del.; Ken- Assistant Examiner'ucE-slm'esser neth A. Scott, Swarthmore, Pa. Att0rneyGeorge L. Church, Donald R. Johnson and Wilmer E. McCorquodale, Jr. [73] Asslgnee: Sun Oil Company, Philadelphia, Pa. 1221 Filed: May 1, 1970 1 ABSTRACT [2]] Appl 33,981 Mixtures of C bicyclic aromatic hydrocarbons containing dimethylnaphthalenes are difficult to fractionate by conventional techniques such as distillation or crystallization. How- [52] U.S.Cl. ..260/674N ever, by contacting such mixtures with the dianhydride f [5 1] Int. Cl ..C07c 7/00 1,2AibenzenetetracarboxYlic acid, the dianhydride of [58] Field of Search ..260/674N 1,23Abenzeneteuacarboxwm acid or the trianhydride f l,2,3,4,5,6,-benzenehexacarboxylic acid, a solid complex of [56] References C'ted certain hydrocarbons and the polyanhydride is formed.

UNITED STATES PATENTS Separation of the solid complex and its subsequent decomposition results in a complexate that 18 substantially richer in 2,941,017 6/ I960 Veatch et al. ...260/674 those dimethylnaphthalenes which are preferentially com- 3,249,644 5/1966 Hahn ..260/674 plexed. Upon further processing these dimethylnaphthalenes have utility in the production of dyes.

1 1 Claims, N0 Drawings FRACTIONATION OF C BICYCLIC AROMATIC HYDROCARBONS BY DI OR TRIANHYDRIDE COMPLEX FORMATION CROSS REFERENCES TO RELATED APPLICATIONS The present application is copending with the following listed applications filed by even date herewith, all applications being of common ownership.

Ser.

No. Inventor(s) Title 33,980 Davis- K. A. Scott Fractionation of Eutectic Mixture of Dimethylnaphthalenes by Dianhydride complexation" "Fractionation of C Bicyclic Aromatic Hydrocarbons by Tetrahalophthalic Anhydride Complex Formation Fractionation of C Bicyclic Aromatic Hydrocarbons by 2- Chloro, 4 Nitrobenzoic Acid Complex Formation" Fractionation of C Bicyclic Aromatic Hydrocarbons by Di or Trianhydride Complex Formation 33,949 R. I. Davis K, A. Scott 33,950 R. I. Davis BACKGROUND OF THE INVENTION This invention relates generally to a process for fractionating difficult-to-separate C bicyclic aromatic hydrocarbons and in particular the isomers of dimethylnaphthalenes. More specifically, it relates to a process for fractionating isomers of dimethylnaphthalene by the initial contacting of the isomers with a di or trianhydride defined hereinafter.

Dimethylnaphthalenes are oxidized to naphthalene-carboxylic acids which are used in the production of dyes and pigments. A more detailed discussion of the utility of dimethylnaphthalenes appears in Naphthalenecarboxylic Acids" by K. A. Scott in Kirk-Othmer, ENCYCLOPEDIA OF CHEMI- CAL TECHNOLOGY, 2nd Edition, Vol. 13.

For convenience dimethylnaphthalene or dimethylnaphthalenes herein will be referred to as DMN, with specific C Alkylnaphthalenes Boiling Point, F.*

API Project 44,Tables 23-2 33.s2oo). 23-2- 33.s2ro and 234 315211 Since 2,6-DMN and 2,7-DMN have the same boiling points, these isomers cannot be separated by distillation. Other such pairs are 1,7-DMN and 1,6-DMN, as well as 1,3-DMN and 1,5-DMN.

Certain DMN isomers can be separated using the differences in their freezing points (see U.S. Pat. No. 3,202,726 issued Aug. 24, 1965 to E. W. Malmberg et al. and US. Pat. No. 3,173,960 issued Mar. 16, 1965 to W. M. Robinson).

However, despite differences in freezing points, DMN form eutectics which make it impossible to further separate certain isomers by crystallization. For example, 2,6-DMN with a freezing point of 234 F. and 2,3-DMN with a freezing point of 221 F. form a eutectic with a freezing point of about 170 F. Thus methods other than crystallization or in addition to must be employed.

Other techniques have been suggested for purification and/or separation of DMN. US. Pat. No. 3,183,279 issued May 11, 1965 to I. W. Mills et all uses selective oxidations. US. Pat. No. 3,155,739 issued Nov. 3, 1964 to G. Suld uses a I-IFzBF complex. Still others have facilitated separation and/or purification by isomerizing various DMN isomers to specific isomers (see G. Suld et al., J. ORG. CHEM., 29,

DMN isomers being indicated by reference to the location of the methyl groups. For example, 2,6-dimethylnaphthalene will be referred to as 2,6-DMN.

DMN are found in coal tar, lignite tar, crude oil, the drip-oil fraction produced during the pyrolysis of hydrocarbons to make olefins, in heavy petroleum reformate and in petroleum gas oil produced by catalytic cracking. In these hydrocarbon mixtures DMN are usually present in rather dilute concentration. For example, one analysis shows DMN making up about 4 percent by weight ofa gas oil. However, by known processes such as distillation, crystallization and solvent extraction, DMN can be recovered in concentrated form from the previously mentioned sources. In oxidizing these DMN to carboxylic acids, it is usually preferable that each isomer by oxidized by itself since generally each isomer requires slightly different reaction conditions for optimum oxidation.

ethyl- 7O Summarizing, to obtain sufficiently pure isomers of DMN by known methods is usually difficult. There is a need for another purification and/or separation method or a method which facilitates existing procedures.

SUMMARY OF THE INVENTION of of of of This invention relates to a method for the fractionation DMN by selective complexation with the dianhydride 1,2,4,5-benzenetetracarboxylic acid, the dianhydride 1,2,3,4-benzenetetracarboxylic acid or the trianhydride l ,2,3,4,5,6-benzenehexacarboxylic acid.

A C bicyclic aromatic hydrocarbon mixture containing DMN is contacted with one of the aforementioned solid polyanhydrides. The resulting solid DMN-polyanhydride complex is separated from the mixture, The solid complex is decomposed and the subsequently released DMN have a composition substantially different from the original hydrocarbon mixture. The composition of the resulting complexate depends on the specific polyanhydride used, the ratio of polyanhydride to DMN, the concentration of the DMN in the initial hydrocarbon mixture, the tendency of the various DMN to form complexes with the polyanhydride, the temperature during the process and the length of time of contacting.

DESCRIPTION The complexing agents used in this invention are the dianhydride of l,2,4,5-benzenetetracarboxylic acid, also known as pyromellitic dianhydride, dianhydride of 1 ,2, 3 ,4- benzenetetracarboxylic acid and the trianhydride of 1,2,3,4,5,6-benzenehexacarboxylic acid. These three polyanhydrides for convenience are referred to herein as PMDA (1), DA (II) and TA (III), respectively. The structures represent ing these three polyanhydrides are as follows:

When solid PMDA, DA or TA is mixed with liquid omit,

the solid visibly increases in volume, generally changes color from white to yellow and gains weight. The generally water white liquid also changes its color to yellow. Very little solid polyanhydride dissolves at ambient temperature or at a higher temperature used in this method.

It is believed that the complexes formed herein are 77 com plexes, i.e., that they are caused by combination between the 1r electrons of the two rings involved. The polyanhydride apparently accepts a share in the 11' electrons of the compound which is complexed with it. Steric factors appear to have a strong effect, since according to the theory of 1r complex formation, the two rings must be close together and parallel in order for the complex to form. These complexes are distinct from the acid-base type as exemplified by complexes of HF :BF and xylenes and also from clathrate complexes of, for example, the urea-paraffin type.

By comparison with said polyanhydrides which will form complexes, the precursor acids corresponding thereto, name- 1y, 1,2,4,5-benzenetetracarboxylic acid, 1,2,13,4- benzenetetracarboxylic acid and l,2,3,4,5,6-benzenetetracarboxylic acid, were found not to form complexes with DMN.

Complexing occurs over a relatively wide temperature range with the rate of complex formation increasing with increased temperature. For example, with a C bicyclic aromatic hydrocarbon mixture complexing takes about four days at F.; whereas the same mixture complexes in about one hour at 77 F. From the foregoing example, it can be seen that the lower temperature limits for the process of the present invention are dictated by practical considerations regarding the rate of complex formation. The upper temperature limits of the process are governed by the thermal stability of the given complex. Thus it is apparent that the optimum temperature for the operation of the present process depends upon both the rate of complex formation and the stability factor. in general the temperature employed will be below the melting point of the polyanhydride. The reported polyanhydride melting points are:

PMDA 540546F. DA 382386F. TA 590F.

Preferably, the temperatures employed will fall in the range from the temperature at which the DMN are a liquid to the melting point of the polyanhydride. However, an inert solvent as defined hereinafter with a freezing point lower than the C, bicyclic aromatic hydrocarbons can be used thereby permitting complexing to take place at lower temperatures.

In addition to finding that complexing occurs over a relatively wide temperature, we have found that the complex formation apparently proceeds in two stages at ambient tempera tures. The first stage is the immediate complexation which apparently is partial and relatively nonpreferential. The second stage is the gradual equilibration with increased preferentiality and completeness. This second stage can be accelerated by in itially heating the mixture of DMN and polyanhydride to a temperature in the preferred range between from 100 F. to 300 F. and then cooling to ambient temperature or even a lower temperature. in this way the equilibration time can be at least halved.

Some DMN isomers complex more readily than others with a polyanhydride. For example, a mixture of said hydrocarbons containing, in addition to other DMN, 14.3 percent of 1,6 DMN and 19.1 percent of l,7DMN is treated with PMDA.

The resulting complexate contains 14.5 percent of 1,6-DMN and 37.5 percent of 1,7-DMN. Comparison of the amounts of 1,7-DMN and 1,6-DMN in the complexate to the amounts present in the initial mixture indicates that the 1.7-DMN in this mixture is preferentially complexed with PMDA compared to the 1,6-DMN. These data are reported in Table 1. Note that because of the same boiling points it would be impossible to distill 1,6-DMN and 1,7-DMN from each other.

in addition, there is some minimum DMN concentration in an inert solvent at which PMDA-DMN complexation will not occur. For example, if a simple mixture of decane and 2,6- DMN at ambient temperature is treated with a molar excess of PMDA, no complex will form if the concentration of 2,6- DMN is less than about one weight percent. Also, if a simple mixture of decane and 2,7-DMN at ambient temperature is treated with a molar excess of PMDA, no complex will form if the concentration of 2,7-DMN is less than about three weight percent. As discussed hereinafter this minimum concentration phenomenon can be used to release the DMN from the formed complex. As to the other polyanhydrides and other DMN and other solvents, this minimum concentration can be determined by the methods discussed hereinafter.

The initial amount of a specific DMN in the mixture affects the percentage increase obtained by complexation. In other words if the initial quantity of a DMN is low, the amount of this DMN found in the complexate is high relative to the initial quantity. Or, if the initial quantity of a DMN is high, the amount of this DMN found in the complexate is only slightly higher than the amount present in the initial quantity. Thus, for example, if a hydrocarbon mixture containing 19.0 percent 1,7-DMN is treated with PMDA, the resulting complexate contains 37.6 percent 1,7-DMN. Table 1 contains additional details. Thus the complexate contains percent more 1,7- DMN than the initial mixture. However, if a hydrocarbon mixture containing 37.6 percent 1,7-DMN is treated with PMDA, the complexate contains 47.6 percent 1,7-DMN, an increase of only 27 percent. Table 11 contains additional details.

Some DMN isomers complex more readily with DA than with PMDA. For example, when a hydrocarbon mixture initially containing 14.3 percent l,6-DMN is treated with PMDA, the resulting complexate contains only 14.5 percent 1,6-DMN (See Table 1). However, if a hydrocarbon mixture initially containing 13.9 percent 1,6-DMN is treated with DA, the resulting complexate contains 23.3 percent 1 ,6-DMN (See Table VI). DA complexes, in additional to individual isomer fractionation, also can be used to make a broad fractionation. Thus almost all the DMN isomers with one methyl group at the a position can be fractionated by complexation from DMN isomers with both methyl groups at B positions.

The feed to this process can include, in addition to at least two DMN isomers, other compounds that do not alter or destroy the structure of the complex. in general, appreciable quantities of undesirable compounds that will react with a polyanhydride are to be avoided. Compounds such as C,, to C alkanes, alkenes, cycloalkanes, cycloalkenes and mixtures thereof were found to be relatively inert and had no appreciable effect upon the complex formation as long as a minimum DMN concentration is maintained. Other inert compounds include CCl. and ethers.

Since this process is for fractionating DMN, hydrocarbons boiling outside the boiling range of C bicyclic aromatic hydrocarbons and which form complexes with the polyan hydrides should not be present in appreciable quantities in the feed. For example a-methylnaphthalene is known to form a complex with PMDA (Bull. CHEM. SC. Japan, Vol. 38, No. 12, pages 2110 to 2ll4, A Study of the Charge-Transfer Complexes ll. The Complexes of Pyromellitic Aromatic Compounds, Taku Matsuo). This problem can be avoided by treating only C bicyclic aromatic hydrocarbons containing DMN. Preferably, the C bicyclic aromatic hydrocarbons are C alkylnaphthalenes and ideally they are only DMN.

The amount of PMDA, DA or TA employed in the complexing step can vary over a wide range depending upon the fractionation desired. The amount of polyanhydride used is related to the amount of DMN present. If an extremely large ratio of polyanhydride to DMN is used and sufficient time allowed, all the DMN would complex and no DMN fractionation could be obtained. On the other hand, the amount of polyanhydride used can be greater than the amount necessary to ultimately form a complex containing all the DMN in the mixture being treated if the length of time of contacting is relativelyshort. Preferably, the amount of polyanhydride contacting the hydrocarbon mixture would be in the range from 0.0l to 3.0 moles of polyanhydride per mole of DMN. A narrower range would be from 0.10 to 15 moles of polyanhydride per mole of DMN. The contacting of the C bicyclic hydrocarbons with a polyanhydride can be performed in one contacting stage or a plurality of distinct contacting stages.

DMN can easily be separated from the complex by heating the latter under vacuum and recovering the DMN as a distillate. By employing such a preferred operation, the polyanhydride is regenerated and can be reused for further complexing. Recovery of the DMN can also be done by elution of the complex with an inert solvent such as decane or destruction of the polyanhydride by such agents as water or an aqueous base.

If an inert solvent is used to elute the complex, a sufficient quantity must be used. The quantity necessary depends on the particular DMN in the complex and the temperature used. Thus if just a 2,6-DMN-PMDA complex is eluted at an ambient temperature with a C to C alkane, such as decane, the amount of decane used must be such that the resulting concentration of 2,6-DMN in the 2,6-DMN-decane mixture is less than 1 wt. percent. However, if the temperature of elution and separation is greater than ambient temperature, the resulting concentration of 2,6-DMN in the 2,6-DMN-decane mixture can be somewhat greater than 1 wt. percent.

Thus in this invention a C bicyclic aromatic hydrocarbon mixture containing DMN is contacted in a liquid phase with solid PMDA, DA or TA. The amount of polyanhydride used is sufficient to preferentially complex with at least one of the DMN, preferably 0.0l to 3.0 moles of polyanhydride per mole of DMN. The temperature of contacting is less than the melting point of the polyanhydride. The temperature of the resulting combination of said aromatic hydrocarbons and polyanhydride can be maintained at ambient temperature, e.g., 50 to 100 F., until the desired or final equilibration is reached. Or, the temperature of the resulting combination after contacting at ambient temperature can be elevated to a higher temperature, the latter being less than the melting point of the polyanhydride to reduce the time required to reach the desired equilibration. The DMN-polyanhydride complex can be separated from the resulting admixture at a suitable elevated temperature although it is preferred to remove the complex after the admixture is at a lower temperature. The lower temperature can be ambient temperature or lower, e.g., 0 F.

Suitable agitation of the C bicyclic aromatic hydrocarbons containing DMN can occur during or after the addition of the polyanhydride and/or during the heating and/or cooling steps.

The solid DMN-polyanhydride complex can be decomposed in several ways. For example, after the solid DMN- polyanhydride complex has been separated, the complex can be heated under vacuum and the DMN removed as distillate. Preferably, the complex should be washed to remove liquid on the surface of the solids. This liquid will have a composition equal to the uncomplexed material and its presence reduces the effectiveness of separation. Another way to decompose the complex is to add a suitable inert solvent such as a C to C alkane, alkene, cycloalkane, cycloalkene and mixtures thereof, as a result of which the complex will decompose. The paraffin should have a boiling point such that it can easily be separated from the DMN by distillation. The solid polyanhydride is removed and the remaining, for example, DMN-alkane mixture fractionated.

An alternative procedure comprises adding a suitable inert solvent and raising the temperature of the resulting complexsolvent combination. Upon decomposition of the complex, the polyanhydride is removed from the hot inert solvent and the remaining DMN-solvent mixture fractionated. In this latter technique, the use of elevated temperatures reduces the necessary amount of solvent.

Still another way to decompose the complex is that the separated DMN-polyanhydride complex is contacted with a compound which will react with the polyanhydride, thereby releasing the DMN. Among such materials are water, aqueous sodium hydroxide, aqueous calcium hydroxide, etc. The advantage of using aqueous sodium hydroxide, etc., is that the formation of a salt which dissolves in the water enhances the separation of the complexate from the water. When the polyanhydride is not reacted with any compound to release the DMN, it can be used to contact untreated C bicyclic aromatic hydrocarbons and/or the complexate and/or the noncomplexate from the first contacting step. This procedure can be repeated as often as necessary to achieve the desired concentration of DMN.

The noncomplexate remaining after one or more solid DMN-polyanhydride complexes have been removed can be contacted with fresh or recycled polyanhydride for further processing according to this invention.

The following examples illustrate this invention:

EXAMPLES I-V In these examples PMDA was used as the complexing agent. It was a white powder with a purity of 98 percent; its melting point was 540 to 546 F.; particle size was percent less than 10 microns; its boiling point was 745 to 752 F. and its specific gravity was 1.68.

The composition of one hydrocarbon mixture treated to illustrate this invention is shown in the following Table I and is referred to as feed.

The feed was treated in the following manner. One mole of PMDA per 5 moles of DMN present in the feed was added to a sample of the feed at room temperature. The resulting mixture was slowly heated to 270 F. and then cooled to 65 to 75 F. and allowed to remain at this lower temperature for 15 to 60 minutes. The mixture of feed and complex was filtered. The solid complex was washed with hexane and vacuum dried. The solid complex was placed in a sufficient amount of decane to decompose the complex and heated up to 270 F. The solid polyanhydride was filtered from the hot decane. The remaining liquid was stripped of the decane to yield the complexate. The following Table I also shows the composition of the complexate and the noncomplexate. Also shown is the weight or mole ratio of a specific DMN in the complexate to the same DMN in the noncomplexate. If this ratio equals one, no change in concentration occurred. If this ratio is greater than How other C bicyclic aromatic hydrocarbon mixtures with different compositions, and referred to as charge, can be treated with PMDA are shown by the data in the following Tables IV and V. The procedure used to obtain the data reported TABLE 1 in Table l was re eated to obtain the data in the followin two P 8 [Formation of DMN-PMDA complexes, first stage] tables Ratio of The data in the preceding Tables I, ll and III also indicates compound how various C bicyclic aromatic hydrocarbon mixtures of Weight percent in commeme, Varlous p ons Complex with PMDA, Th Same dam Com- Noncom Noncomalso illustrates that with a PM DA multistage complexation the Compounds Feed Pemte plum plexam degree of separation decreases as the number of stages in Biphenyl methylnapthalcnes $.2 3 9 29.9 creases. Others: (12 th 1 hth 1 8 v 3 s 8 0 4 l-an -e ynap acne" .7 TABLE Iv 2- g l 5 [Formation of DMN-PMDA complexes] l;? 1l. $13 $32 iii? 5:53 E'j 9fi1 w 3 l5-DMN 1.0 2.0 0.5 4.00 N I l-DMN 1 3 o 6 1 4 o 42 comp '3 13' 6 21 6 13' 1 1' 6 COIN- COlIl- IO lIOIlCOlll- 2:DMN:::: 0.6 0.3 0: (,ompounds Charge plexatc plexato plexatc i LOO 2 Othcal'ogliatics 53,2 23 3 &2

1- an 2-e y no. hthalenc 4,5 ,9 Total 100-0 0 0 100 0 2,6-DMN f M 31, '18 ff; 2,7-DMN 4.8 5.2 4.5 1,2 1,6-DMN. a. 8.0 12. 4 6. 9 1. 8 The complexate from Table l was again treated with one 10. J 26.3 6.6 4.0 mole of PMDA per 5 moles of DMN and the procedure 25 ,4 DM 0.5 0.8 0.5 1.6 described for the first complex formation repeated. The analy- 8.2 17. 6 5. 4 3. 2 sis of this second complexate and the second noncomplexate I: 3;; 11% 81% :3 IS shown in the following Table I] along with the analysis ofthe first complexate Total r a a r r v 100.0 100 0 100 0 TABLE II [Formation of DMN-PMDA complexes, second stage] Ratio of compound Weight percent in second comploxato First Second Second nonto second Compounds comploxato complexato complcmto noncomploxato Otherarornatics a. 3.9 1.5 6.3 ,1 a land 2-cthylnaphthalcnc 3. 7 1.3 5.0 0. 27 MN 10. 1 11.3 (a. 1 1. 6O

4. 5 1.1, 5. s 0. 2 .1 4 14. 5 10. s to. a 0. (i5 37. o 47. 1, 34. o 1. 2.0 3. 5 1.4 2. 0.6. 0.5 21.11 23 5 21.4 1.10 0. 3 0. z 1. 2 0. 11 1. o 0. 110

The second complexate from Table II was again treated 50 with one mole of PMDA per 5 moles of DMN and the procedure described for the first and second complexations repeated. The analysis of this third complexate and the third noncomplexate is shown in the following Table III along with the analysis of the second complexate.

TABLE III [Formation of DMN-PMDA complexes, third stage] Ratio of compound in third Weight percent complexate to Second Third Third third Compounds complexate Complexatc noncomplexate noncomplcxatc Other aromatics l. 1.5 Trace 1. T l-and-2-ethylnaphthalene. 1. 3 O. 4 2. 1 0.3 2,6-DMN J. 3 6. 8 1. 0 2,7-DMN.- .6 2. 6 0. 5 1 6-DMN. .8 10. 8 0. 6 1,7-DMN. .6 47.3 1. 2 1,5-DMN .5 3. 6 1. 5 1,4-D MN 0. 3 1,3-DMN 23. 6 0. 9 12-DMN 2,3-DMN 1. 2

Total 100.0 100. 0 100. 0

TABLE [Formation of DMN-PRIDA complexes] Weight percent Ratio of compound in New complexato Comcom to noncn1- Compounds Charge plexato plcxate ploxate Other aromatics 66. t) 26. .2 74. 6 l-and 2-ethylnaphthalene 3. 1 2. 2 3. 1 0.71 iii-DMN.... 1.3 3.! 0.8 4.30 8. 8 1O. 6 8. 2 1. 30 4. 3 ti. 2 3. J 1. 60 4. 3 11. 7 3. 0 3. 00 7. 0 .29. 4 3. 0 J. 80 0. .2 0. 5 0. 3 1 7O 3. 7 J. 0 2. 7 3. 30 0. 1 0. 2 0. 1 2. 00 0. 3 0. 6 0. 3 l. 00

EXAMPLE VI In a manner similar to that described for PMDA, a sample of the same feed which was used to obtain the data reported in Table I was treated with one mole of DA per 5 moles of DMN. The DA used was a white powder and had a melting point of 382 to 386 F. The following Table VI shows the composition of the compiexate and the noncomplexate. Comparison of the results reported in Table VI with those reported in Table I clearly shows that DA and PMDA have different complex preferences with DMN. Shown also is the weight or mole ratio of a specific DMN in the complexate to the same DMN in the noncomplexate.

TABLE VI [Formation of [MIN-DA complexes] Weight percent Ratio of compound Nonin complex- Comcomate to non- Compounds Feed plexate plexato comploxato Biphenyl methylnaphthaleues 2S. 0 8. 3 27. 1 Others:

1- and lethylnaphthalene S. 0 3. 1 8. 2 0. 38

2,7-DMN.. 8.3 5.7 8.3 0.68

1,6-DMN. 13. 9 23. 3 14. 3 1. 60

1,T-D1\lN 18. 8 34. 5 19. 1 1. 8

1,5DMN 0. 6 0.8 1.0 0.8

1,4-D.\IN 1.1 1.4 1.3 1.1

1.3-Di\1N l4. 6 19. 0 13. G 1. 4

LS-DMN... .1. 1.0 1.1 1.3 0.9

'lOtuL 100.0 100.0 100. 0

The data in Table VI indicate that most of the DMN with one methyl group in the a-position are preferentially complexed by DA compared to the DMN with both methyl groups in a B-position, particularly 2,6-DMN and 2,7-DMN.

EXAMPLE VII To illustrate that an inert solvent could be used to dilute a C bicyclic aromatic hydrocarbon mixture without substantially affecting complexation, the following two runs were made. Said hydrocarbon mixture containing 5.0 percent 2.6- DMN and 8.2 percent 2,7-DMN by itself and said mixture diluted with decane, each was treated as shown in the following Table VII. The addition of a weight decane equal to 2.5 times the weight of said hydrocarbon mixture did not substantially affect the composition of the complexate.

TABLE VII Comparison of Complexation With and Without Inert Solvent Without Decune With Decune Amount of DCLUIIC Used None Weight of decane equal to 2.5 times weight of hydrocarbon mixture Moles of PMDA/ mole of DMN 0.25 0.22 C bicyclic aromatic hydrocarbon mixture containing:

2,6-DMN 5.0% 2,7-DMN 8.2% Complexate 2,6-DMN 9.6% 9.37r 2,7-DMN 4.7% 4.3% Noncomplexate I 2,6-DMN 4.5% 3.8 7H 2,7-DMN 8.6% 9.0%

Analyses shown on decanefree basis.

EXAMPLE VIII The foregoing examples illustrate how PMDA and DA formed complexes with various DMN. Typically, the amounts of DMN complexed per 100 grams of PMDA and DA were 60 to grams and 25 to 35 grams, respectively. However, when TA was used only 5 to 7 grams of DMN were complexed per grams of TA indicating a much lower effectiveness. The TA used was a white powder and had a melting point greater than 590 F.

We claimed 1. A method of fractionating a mixture of C bicyclic aromatic hydrocarbons containing dimethylnaphthalenes comprising:

a. contacting said mixture in a liquid phase with a solid complexing polyanhydride selected from the following group: the dianhydride of l,2,4,S-benzenetetracarboxylic acid, the dianhydride of l,2,3,4-benzenetetracarboxylic acid and the trianhydride of l,2,3,4,5,6-benzenehexacarboxylic acid; at a temperature below the melting point of the polyanhydride to complex preferentially with at least one of the dimethylnaphthalenes and form a solid complex containing less than the total amount of dimethylnaphthalenes in said mixture;

b. separating the solid complex from the resulting admixture;

. and decomposing the solid complex to recover the resulting complexate having a proportion of dimethylnaphthalenes different from that in the starting hydrocarbon mixture.

2. A method according to claim 1 wherein the mixture of hydrocarbons is contacted with the polyanhydride at 50 to 100 F. after which the temperature of said resulting admixture is increased to within the range from 100 to 300 F. after which the solid complex is separated at 0 to 100 F.

3. A method according to claim 1 wherein the temperature at which the polyanhydride is contacted with said mixture is in the range from 100 to 300 F. and the solid complex is separated at 0 to l00 F.

4. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of C alkylnaphthalenes containing dimethylnaphthalenes.

5. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of dimethylnaphthalenes.

6. A method according to claim 1 wherein the amount of polyanhydride contacting the mixture is in the range from 0.01 to 3.0 moles per mole of dimethylnaphthalenes.

7. A method according to claim 1 wherein the mixture of said hydrocarbons consists essentially of a mixture of C alkylnaphthalenes containing dimethylnaphthalenes. The contactin polyanhydride is selected from the followin group: dianhy ride of l,2,4,5-benzenetetracarboxylic acid and dian 8. A method of fractionating a mixture of dimethylc. and decomposing the solid complex to recover the resultnaphthalenes comprising: ing complexate having a proportion of dimethyla. contacting said mixture in a liquid phase with 21 solid comnaphthalenes different from that in the starting dimethylplexing polyanhydride selected from the following group: naphthalene mixture the dianhydride of l,2,4,5,-benzenetetrucarhoxylic acid, 5 9. A method according to claim 8 wherein the solid comthe dianhydride of l,2,3,4-benzenetetracarboxylic acid plexing polyanhydride is the dianhydride of 12,45- and the trianhydride of I ,2,3,4,5 ,6-benzenehexacarboxylbenzenetetracarboxylic acid.

ic acid; at a temperature below th elting oint f th 10. A method according to claim 8 wherein the solid compolyanhydride to complex preferentially with at least one plexing polyanhydride is the dianhydride 0f .2. of the dimethylnaphthalenes and form a solid complex 10 nz ne etrucarboxylic acid.

containin l ss than th total amount of di mh l- 11. A method according to claim 8 wherein l,7-dimethylnaphthalenes in said mixture; naphthalene complexes preferentially to the other dimethylb. separating the solid complex from the resulting admix naphthalenes.

lure; n- -0- u- 

2. A method according to claim 1 wherein the mixture of hydrocarbons is contacted with the polyanhydride at 50* to 100* F. after which the temperature of said resulting admixture is increased to within the range from 100* to 300* F. after which the solid complex is separated at 0* to 100* F.
 3. A method according to claim 1 wherein the temperature at which the polyanhydride is contacted with said mixture is in the range from 100* to 300* F. and the solid complex is separated at 0* to 100* F.
 4. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of C12 alkylnaphthalenes containing dimethylnaphthalenes.
 5. A method according to claim 1 wherein the mixture of hydrocarbons consists essentially of a mixture of dimethylnaphthalenes.
 6. A method according to claim 1 wherein the amount of polyanhydride contacting the mixture is in the range from 0.01 to 3.0 moles per mole of dimethylnaphthalenes.
 7. A method according to claim 1 wherein the mixture of said hydrocarbons consists essentially of a mixture of C12 alkylnaphthalenes containing dimethylnaphthalenes. The contacting polyanhydride is selected from the following group: dianhydride of 1,2,4,5-benzenetetracarboxylic acid and dianhydride of 1,2,3, 4-benzenetetracarboxylic acid. The amount of dianhydride contacting the C12 alkylnaphthalenes is in the range from 0.1 to 1.5 moles per mole of dimethylnaphthalenes, the temperature at which the dianhydride is contacted with said alkylnaphthalenes is in the range from 100* F. to 300* F. and the solid complex is separated at 0* F. to 100* F.
 8. A method of fractionating a mixture of dimethylnaphthalenes comprising: a. contacting said mixture in a liquid phase with a solid complexing polyanhydride selected from the following group: the dianhydride of 1,2,4,5,-benzenetetracarboxylic acid, the dianhydride of 1,2,3,4-benzenetetracarboxylic acid and the trianhydride of 1,2,3,4,5,6-benzenehexacarboxylic acid; at a temperature below the melting point of the polyanhydride to complex preferentially with at least one of the dimethylnaphthalenes and form a solid complex containing less than the total amount of dimethylnaphthalenes in said mixture; b. separating the solid complex from the resulting admixture; c. and decomposing the solid complex to recover the resulting complexate having a proportion of dimethylnaphthalenes different from that in the starting dimethylnaphthalene mixture.
 9. A method according to claim 8 wherein the solid complexing polyanhydride is the dianhydride of 1,2,4,5-benzenetetracarboxylic acid.
 10. A method according to claim 8 wherein the solid complexing polyanhydride is the dianhydride of 1,2,3,4-benzenetetracarboxylic acid.
 11. A method according to claim 8 wherein 1,7-dimethylnaphthalene complexes preferentially to the other dimethylnaphthalenes. 